Why is it wrong to write "Vreau pantaloni aceiaşi"?
I think right way would be Vreau pantalonii aceiaşi. pantalonii with two ii. because when aceiaşi is after word you should use definite article.
Actually, no, același, celălalt etc. always go before the noun.
Why not "Vreau același pantaloni"
"Pantaloni" is masculine and plural, so you need also to put the adjective of the same number and gender: "aceiași pantaloni", while "același" corresponds to masculine gender but it is singular.